Why is the Sun’s corona so hot? Two words: Solar tornadoes

These volcano-ish vortices in Sun's atmosphere may be from twisted mag. fields.

Five different wavelengths of light reveal a vortex of gas in the Sun's atmosphere. This swirling gas may be the result of intense magnetic fields and may explain why the Sun's corona is so hot.

Scullion, Wedemeyer-Böhm et al.(2012); Background image: NASA

One of the abiding mysteries surrounding our Sun is understanding how the corona gets so hot. The Sun's surface, which emits almost all the visible light, is about 5800 Kelvins. The surrounding corona rises to over a million K, but the heating process has not been identified. Most solar physicists suspect the process is magnetic, since the strong magnetic fields at the Sun's surface drive much of the solar weather (including sunspots, coronal loops, prominences, and mass ejections). However, the diffuse solar atmosphere is magnetically too quiet on the large scales. The recent discovery of atmospheric "tornadoes"—swirls of gas over a thousand kilometers in diameter above the Sun's surface—may provide a possible answer.

As described in Nature, these vortices occur in the chromosphere (the layer of the Sun's atmosphere below the corona) and they are common. There are about 10 thousand swirls in evidence at any given time. Sven Wedemeyer-Böhm and colleagues identified the vortices using NASA's Solar Dynamics Observatory (SDO) spacecraft and the Swedish Solar Telescope (SST). They measured the shape of the swirls as a function of height in the atmosphere, determining they grow wider at higher elevations, with the whole structure aligned above a concentration of the magnetic field on the Sun's surface. Comparing these observations to computer simulations, the authors determined the vortices could be produced by a magnetic vortex exerting pressure on the gas in the atmosphere, accelerating it along a spiral trajectory up into the corona. Such acceleration could bring about the incredibly high temperatures observed in the Sun's outer atmosphere.

The Sun's atmosphere is divided into three major regions: the photosphere, the chromosphere, and the corona. The photosphere is the visible bit of the Sun, what we typically think of as the "surface." It exhibits the behavior of rising gas and photons from the solar interior, as well as magnetic phenomena such as sunspots. The chromosphere is far less dense but hotter; the corona ("crown") is still hotter and less dense, making an amorphous cloud around the sphere of the Sun. The chromosphere and corona are not seen without special equipment (except during total solar eclipses), but they can be studied with dedicated solar observatories.

To crack the problem of the super-hot corona, the researchers focused their attention on the chromosphere. Using data from SDO and SST, they measured the motion of various elements in the Sun's atmosphere (iron, calcium, and helium) via the Doppler effect. These different gases all exhibited vortex behavior, aligned with the same spot on the photosphere. The authors identified 14 vortices during a single 55-minute observing run, which lasted for an average of about 13 minutes. Based on these statistics, they determined the Sun should have at least 11,000 vortices on its surface at any given time, at least during periods of low sunspot activity.

Due to the different wavelengths of light the observers used, they were able to map the shape and speed of the vortices as a function of height in the chromosphere. They found the familiar tornado shape: tapered at the base, widening at the top, reaching diameters of 1500 km. Each vortex was aligned along a single axis over a bright spot in the photosphere, which is the sign of a concentration of magnetic field lines.

With that hint, they ran a series of computer simulations including both gas and magnetic effects. They found the processes in the photosphere funneled the magnetic field lines into knots of high concentration, which rotate as the gas itself circulates. (This is known as the "bathtub effect," since it rotates in a similar way to the vortex produced in a draining tub of water). The intensity of the magnetic field in turn forces the gas in the chromosphere to follow a spiral trajectory up and away from the Sun's surface, speeding it up during the process. Temperature is related to the average speed in a gas, so higher speeds mean higher temperatures—sufficient to explain the million-degree coronal temperatures.

The number of observed vortices is small, but the results from the current study are sufficiently interesting to merit follow-up observations. If the model holds up under further investigation, it could solve the problem that has plagued astronomers since they first characterized the corona in 1869.

Most solar physicists suspect the process is magnetic, since the strong magnetic fields at the Sun's surface drive much of the solar weather (including sunspots, coronal loops, prominences, and mass ejections). However, the diffuse solar atmosphere is magnetically too quiet on the large scales.

Last I recall magnetic reconnection was touted as the explanation for the high coronal temperature.

Swedish space physics kicks some ass! [Disclaimer: I know some of the guys, and I'm a swede.]

The reason seems to be the position to be both fascinated by and having access to polar regional phenomena like auroras within parts of the nation. The summer sun vs winter dark makes for a persistent obsession with the sun too.

@ Dermoplasm/AnonymousRich:

From the description this seems to be a new observation, but I dunno really. Surely similar structures should have been considered as heating gases before now. The thing seems to be that these structures are predicted to suffice by themselves.

Magnetic reconnection is, last I heard, also predicted to suffice. But both mechanisms can be, quite likely are, operating. For example, these magnetic twists could induce some reconnection too, since it would be a way they can relax fast.

The battle of the theories will surely begin or continue for a while. [/brings some popcorn to go with the beer]

---------------------Yeah, yeah, just complain about the high temperatures in US: as a result we have had a summer month that has been the coldest and wettest in 20 years. (No, really, I hear they just figured that the atmosphere is the main driver for east and west facing coastal climates from the equator and way up. So there is a connection.)

An expected local weather extreme under a climate regime of greenhouse warming, and of course more and worse of them as it accelerates, making it feel even worse.

But I hear that the US oddball litigious system just now finally allows for the government agencies to recognize and act on such phenomena - at least by lower courts. And since US is the worst AGW offender by capita, maybe you will start take proportional responsibility any decade now. (O.o)

I don't know. 11,000 tornados on something as big as the sun seems like a low number, no matter how big the vortex. Granted... I don't know and probably can't do the math it takes to figure that out but it SEEMS like a low number.

I don't know. 11,000 tornados on something as big as the sun seems like a low number, no matter how big the vortex. Granted... I don't know and probably can't do the math it takes to figure that out but it SEEMS like a low number.

Tornado is probably a misnomer. Its more like a hurricane that is the size of Alaska with wind speeds in the 1000s of mph. (yes this is just as bad of a mischaracterization)

I don't know. 11,000 tornados on something as big as the sun seems like a low number, no matter how big the vortex. Granted... I don't know and probably can't do the math it takes to figure that out but it SEEMS like a low number.

Tornado is probably a misnomer. Its more like a hurricane that is the size of Alaska with wind speeds in the 1000s of mph. (yes this is just as bad of a mischaracterization)

Let's do the math...According to Wikipedia, the sun has a surface area of 6.0877*10^12 sq. km.

If we assume that the vortex's are on average, 1000 km in diameter and are generally circular in nature, then the "average" vortex covers about 785,000 sq. km. If we then assume that there are 10,000 vortex's at any given time, the total "average" surface area of the vortex's is 7.85*10^9 sq. km.

This calculates to about .13% of the total surface area of the sun has a vortex over it at any given time. While this seems small, it is still 15 times the total surface area of the earth (5.1*10^8 sq. km.)!

Do Qatar and Trinidad and Tobago also take proportional responsibility (whatever that means)?

T&T doesn't take responsibility for picking up their own trash from the streets. So... no.

Of course if they did, it wouldn't make much difference overall. That's why both total pollution and per-capita should be considered. Total because it tells you the scale of the problem. Per-capita because it tells you how excessive the problem is.

Anyway, awesome research about the sun!

I'm disappointed I haven't heard anyone claiming this proves Plasma Cosmology... or are they gone now, maybe after having read an actual book on electromagnetisms?

Maybe trying to small scale this here would be a way to test more of this... I know some of it is on the verge of Star Trekness, but look at all the stuff we have because of ST. Not long to getting dilithium chambers....

Do Qatar and Trinidad and Tobago also take proportional responsibility (whatever that means)?

T&T doesn't take responsibility for picking up their own trash from the streets. So... no.

Of course if they did, it wouldn't make much difference overall. That's why both total pollution and per-capita should be considered. Total because it tells you the scale of the problem. Per-capita because it tells you how excessive the problem is.

Total is all that matters WRT to AGW near as I can tell. If you know of a per-capita term in climate models I'd love to hear about it.

And per-capita seems to tell you how industrialized a country is. When the USSR's economy collapsed years ago they ratified Kyoto shortly after, seems they had some carbon credits they wanted to sell.

Quote:

Anyway, awesome research about the sun!

I concur. Too bad there's not even a decent abstract from the link above, but the movies are interesting. From a few watches they don't seem to show constant rotation (except possibly for what are presumably field lines), so I'd say that they are tornadic more in shape than function. But still, very cool.

Quote:

I'm disappointed I haven't heard anyone claiming this proves Plasma Cosmology... or are they gone now, maybe after having read an actual book on electromagnetisms?

Total is all that matters WRT to AGW near as I can tell. If you know of a per-capita term in climate models I'd love to hear about it.

Total is what matters as to the impact on climate. Both matter as to targeting areas in which to fix the problem, Areas with low total output won't have a big effect even if completely eliminated, and areas with low per-capita output have little excess to optimize away.

It's like any optimization problem for efficiency -- you can't just look at the biggest consumer and say "that needs to go away" because some of it may be necessary. You need to find where there is excess usage to be shaved. Per-capita emissions is a reasonable metric to use for starters.

Do Qatar and Trinidad and Tobago also take proportional responsibility (whatever that means)?

Yes, I am pretty sure, with or without ranking homilies. That is a list of direct emissions* (by sloppy practices such as gas burn off and accidents), while total AGW (and a total account for responsibility) is summed up by looking at consumers. I.e. an oil consumer in a non-oil nation is why some of those emissions occur, a consumer is producing them.

US use ~ 24 % of the produced oil.

I haven't in fact checked my claim, as it is "well known". But US placing ~ 10th place on a production list would mean it is up in the top of producing AGW emissions, per capita.

Proportional responsibility should be fairly obvious. But if not, it would mean "if you are outputting ~ 24%*7*10^9/300*10^6 ~ 6 times as much AGW gas as the average world citizen, you would have to work against outputting 1/6 of whatever we can live with**, or in other word a US citizen would have to reduce his AGW production to roughly ~ 1/6*0.2 = 3 % of his/hers emissions of today".

--------------* Politically intended, I note. Which is a sure sign you shouldn't use them in the first place.

** I was using the 80 % average reduction for a max +2 degC decrease needed before 2020 or so, which is what I remember, in order to be explicit. You can always argue that you don't need to target that safe zone.